4 wire (Kelvin) resistance measurement tutorial


Uploaded by Afrotechmods on 06.07.2011

Transcript:
In this video I'm going to show you how you can use a cheap multimeter to
measure resistances the milliohm range with a decent amount of accuracy.
Most of the time a resistance of a few milliohms doesn't matter.
But if there are high currents in your circuit a few milliohms can cause a
substantial voltage drop and power loss in the wires.
For example let's say you're designing a coilgun or winding a solenoid or you
have a project with long battery wires and you want to know the exact
resistance of a piece of wire.
If you try to use your multimeter's resistance function you'll
get a useless reading.
Zero ohms... 0.3 ohms... I have no idea what's going on here.
Even an excellent multimeter will just say something like 0.2 ohms
when I know for sure the resistance is a lot less than that.
Part of the problem is that you're measuring the resistance of the multimeter's
cables and part of it is that most multimeters are not designed to
measure resistances this small.
The solution to the problem is called four wire measurement, also sometimes
called Kelvin resistance measurement. And it's really simple.
Let's start out with a constant current source of one ampere, and I'm going to show you
how to set that up later.
Since it's a constant current source,
no matter what resistance I connect it to, the current flowing through the circuit
will always be exactly one ampere.
Now let's say I connect an unknown resistance and let's call that
resistance "R".
And let's connect a multimeter across the resistance and use it to
measure the voltage across R.
And let's call that voltage "V".
Ohm's law is Volts = Current x Resistance
and rearranged that gives you Volts divided by Current = Resistance
Now because we know the current in the circuit is exactly one ampere you end up
with V = R.
In other words
the resistance in ohms is the same as the number of volts across R.
So let's say you measure fifty millivolts across the wire you are testing...
that means that the resistance of the wire is fifty milliohms.
Here I am measuring the voltage across a small piece of wire and since the
reading is fourteen millivolts I know that the wire has a resistance of
fourteen milliohms.
You will never get this kind of accuracy with the
traditional two wire measurement approach.
Now another cool thing about this circuit is how the resistances of the test
wires and alligator clips are not a problem anymore.
The current is always exactly one ampere so as long as I measure the voltage
directly across the device under test I can calculate its resistance.
And because the input impedance of the multimeter is so high (well into the
megaohm range) almost no current flows into the multimeter so the meter
doesn't affect the circuit.
Okay now here is the easiest way to do this in real life.
I strongly recommend that you have a bench power supply with current limiting
functionality.
If you don't have one you should check out www.MPJA.com
Start out by setting the voltage to about two volts.
The exact number doesn't matter but keep it a low voltage because we're going to
start short circuiting things soon.
Start with the current limiting dial set to zero.
Now connect the multimeter set to measure current directly across the
power wires.
Now we just need to increase the current limit so that we get exactly one ampere.
Doing this with the multimeter gives you a little more accuracy than just reading
the bench supply's display.
Okay now we have a constant current supply of one ampere.
And you just need to connect your current supply wires to the thing you
want to measure,
then measure the voltage across it.
Let's measure the resistance of this inductor.
47 millivolts means the inductor has a D.C. resistance of
47 milliohms.
And this piece of speaker wire has a resistance of seven milliohms.
Okay now what if you don't have a current limiting power supply like I do?
Well you can build a constant current source out of an LM317
using this circuit.
In theory this should give you a constant current of one ampere regardless of
what you have connected to it.
In reality you'll never find a precise 1.25 ohm
power resistor.
So the current won't be exactly one ampere.
So here's what you have to do.
First use your multimeter to measure the actual current that you're getting out
from the circuit.
In my case I got 0.92 Amps.
Next you measure the voltage across your wire as before.
Let's say it's 80 millivolts for a certain piece of wire.
Now you apply Ohm's law again
but since the current isn't exactly one ampere, you can't do the
easy volts to ohms conversion.
Instead, take your measured voltage and divide it by your measured current and you'll
get the exact resistance of your piece of wire.
That's it for four wire measurement and now your death ray designs can be
more efficient than ever!